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One of the most common email questions I get on solar space heating goes something like:

If I install X sqft of solar collector on my house, what fraction of my heating bill will go away?


Or, How much solar collector area do I have to install to heat my home?


Or, How much of your heating bill does the Solar Shed cover?


These are all good questions, but are not easy to answer.


The first two questions depend on: 1) how much sun you get where you are, 2) how cold your local climate is, 3) how big your house is, 4) how well your house is insulated and sealed, 5) what kind of collectors you install and how they are aimed.   And a few other things, but these are the main ones.


Even if you tell me  all these things, its a pretty lengthy analysis to get to an answer, and I just can't do this for everyone.  Up until now, I have some rough rules of thumb I use to give people a rough idea -- these are better than nothing, but a long ways from good.


On the third question, I can give a rough answer, but it really does you no good to know what my system with my sun conditions and my (very cold) climate does for me.  Installing my system at your house will almost certainly have very different results.



A Simple, But Fairly Good Estimating Procedure

So, getting an idea how much a solar heating system will help out on your heating bill basically involves two things:

- Knowing how much heat you need to heat your house (your house heat loss)


- Knowing how much heat your proposed solar heating system will produce.


House Heat Loss

You can estimate your house heat loss a couple ways. 


You can use this simple Home Heat Loss Calculator to estimate the heat loss for your home.  The calculator will give you a rough estimate of the total heat energy you need to heat the house for a full season.  It will also give you the heat loss per hour for a given outdoor temperature. 


You will need to know the basic geometry of your home (wall areas, ceiling areas, window sizes,...) and the levels of insulation (R value) for each of these components.  While gathering this information takes a little work, it is pretty easy stuff to come by or make reasonable estimates on.  On the plus side, getting the information and running the calculator will tell you which areas of the house (walls, windows, ...) are losing the most heat, and might be worth improving. 


The example will go through this stuff.



Solar Heating System Heat Output

This has been the difficult part to estimate well.  The heat output from a set of solar collectors depends on how much sun they get, how cold it is, how the collectors are aimed, and the characteristics of the collector itself.  Andy's new Solar Collector Power Output simulator gives you a simple way to get a rough idea what the energy output for a set of collectors is.


You can enter your location, the type of collector you plan to use, the collector orientation, and the average storage tank temperature, and the calculator will give you a day by day plot for the full year.


You can use this plot to make a pretty good estimate of the heating season output for your set of collectors.

Andy's simulation takes is an hour by hour simulation for a full year using typical weather for the location you select, so it takes into account the actual solar radiation levels (including effects of cloudy weather) and the actual outdoor temperatures.  This information is used along with the efficiency curve of the collector you select to estimate the heat output of the collector for each hour. 


Some things are not accounted for, including the variation in storage tank temperature, and other losses in the system.  But, it beats the heck out of things like estimating the number of equivalent full sun days that simpler methods use.



You can



An Example







Other Stuff

One thing you are going to quickly determine if you have a typical US house is  that house heat loss is the big player.  If you want your solar heating system to be effective you MUST get your house heat loss as low as you can reasonably manage.  Things like adding insulation, sealing infiltration leaks, sealing heating ducts, window treatments to reduce window heat loss,  etc.  are very important.  Doing these things will normally be more cost effective that doing a solar heating project, and they will allow a smaller solar heating system to cover more of your heating bill.



Do I Need to Do This?

One approach to solar heating systems goes something like this:

- If I put a solar heating system in, its going to give me "free" energy, and its going to reduce my greenhouse gas emissions -- all good.


- In climates that are cold or coldish and with typical home insulation levels in the US, its nearly impossible to put too much collector area in, so I'll just put in as much as my budget and my house geometry allows.  I don't really need to know what fraction of my heating this is going to cover -- whatever it covers will be good.

I don't see anything at all wrong with this line of reasoning.  It makes perfect sense to me, and it lets you get on with things.

I think this makes even more sense with self built systems, as the return on investment is likely to be good -- you won't end up spending a fortune for a system that does not save much.


But, the engineer in a lot of use wants to know in advance what we are going to get for what we are going to spend. 


The advantages of doing the estimate are:

- It gives you a rough idea what you will save in heat bills and carbon emissions to compare to what you will spend. 


- It allows you to compare the savings for a solar heating system to the savings for other energy reduction projects.


- Going through the estimate gives you a better feel for how the whole thing works, and may end up changing the design of the system.


So, to some degree, its a personality thing.  Either way can be right for you.





Gary March 11, 2010